Air conditioning systems for commercial and industrial buildings typically employ water as a heat transfer medium for transferring heat from within the building to an evaporative cooling tower outside the building. The heat transfer water is cooled within the cooling tower by being sprayed over evaporation condensers that evaporate a portion of the water to outside air. The remaining heat transfer water is recirculated through the air conditioning system.
Evaporation condensers are usually exposed to unfiltered outside air. As a consequence, various impurities or contaminants such as, for example, dust, algae, fungus and bacteria, can enter the recirculated heat transfer water. One contaminant that is common to cooling water systems is the bacteria Legionella pneumophila. This bacteria is particularly undesirable because it can sometimes cause various health problems for persons in the building treated by the air conditioning system. Moreover, the bacteria can thrive in the heat transfer water because it is maintained at a temperature of between, for example, 80.degree.-105.degree. F. (26-40.degree. C.).
Biocides are added to many cooling water systems to control L. pneumophila and other bacteria. Biocides are available in liquid or solid form. Liquid biocides (e.g., Product 490 and Product 480 sold by Chemex of Portland, Oregon) are undesirable because they are dangerous to handle and store. In contrast, solid biocides, such as 1-Bromo-3-chloro-5, 5-dimethylhydantoin (BCDMH) distributed in tablet form as Product 492 by Chemax of Portland, Oregon are relatively safe to handle because of their generally inert form.
One method of adding solid biocides to the heat transfer water is to run a small constant flow of make-up (i.e., fresh) water over the biocide tablets and into the cooling tower sump. This method is undesirable because it causes biocide-laden water to overflow the sump when the air conditioning system is off and applies insufficient biocide when the system operates at conventional loads.
In another method, solid biocide is held in a perforated container that resides in the sump of the cooling tower and allows the biocide to dissolve continuously into the heat transfer water. Such an apparatus is undesirable, however, because it does not control the concentration of dissolved biocide in the heat transfer water. For example, a concentration of BCDMH greater than about 12 parts per million of residual chlorine can result in corrosion of many components of the cooling water system. Such concentrations are easily exceeded by this method.